The dominant role of Mn²⁺ additive on the electrochemical reaction in ZnMn₂O₄ cathode for aqueous zinc-ion batteries

Among zinc-ion battery (ZIB) cathodes, ZnMn₂O₄ (ZMO), with its high theoretical capacity and voltage, is an intriguing choice. In this study, we compared the electrochemical activity of a ZMO microrods cathode obtained through a simple co-precipitation process in the presence of a 0.1 ​M MnSO₄ (MS) solution as a full-time electrolyte, as an additive in zinc sulfate (ZMS) electrolyte (1 ​M ZnSO₄ ​+ ​0.1 ​M MnSO₄) and in its absence or a full-time zinc sulfate (ZS) electrolyte (1 ​M ZnSO₄), respectively. Systematic investigations including ex situ X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) studies revealed the reasons for the superior stability and high reversibility of ZMO in the ZMS electrolyte medium. The exceptional performance was facilitated by the electrochemical equilibrium between Zn²⁺ and Mn²⁺ ions via a stable Zn²⁺ (de)insertion in the bulk, a reversible electro-deposition/dissolution of MnO_x from the Mn²⁺ additive in the electrolyte onto(from) the surface of the cathode and the reversible Zn-insertion into the undissolved surface MnO_x layer. This finding is significant as it is contrary to the conventional understanding that the addition of Mn²⁺ merely tends to prevent manganese dissolution thereby facilitating a stable cycle-life performance of the cathode in ZIBs.